This invention relates to flue gas cleanup and, more particularly, to removing nitrogen oxides, sulfur oxides, and particulates from a gaseous stream, such as from a regenerator in a catalytic cracking unit.
Flue gases emitted in combustors, such as in regenerators and power plants, often contain undesirable levels of sulfur oxides (SOx), nitrogen oxides (NOx), and particulates which, if untreated, might pollute the atmosphere.
Sulfur oxides in the presence of water can form sulfuric acid causing acid rain. Nitrogen oxides may cause smog by photochemical reaction with hydrocarbons in the atmosphere. Particulates in flue gases typically include ash (soot) and/or spent combusted catalyst with trace metals, such as arsenic and other contaminants which, in excessive levels, could poison vegetation and livestock.
Over the years, various methods have been suggested for controlling and/or removing sulfur oxide and/or nitrogen oxide emissions. In catalytic cracking units, sulfur oxide control processes usually occur in the regenerator. In one widely used process, sulfur oxides are captured in the regenerator with sulfur oxide-capturing acceptors and subsequently converted to hydrogen sulfide in the reactor and the hydrogen sulfide is withdrawn with the product stream from the reactor and treated in a sulfur recovery plant. Some of the methods suggested for removing nitrogen oxides in regenerators, however, poison the cracking catalyst and are, therefore, unacceptable. Typifying these prior art methods for controlling sulfur oxide and/or nitrogen oxide emissions are those described in U.S. Pat. Nos. 2,493,218; 2,493,911; 2,522,426; 2,575,520; 2,863,824; 2,992,895; 3,023,836; 3,068,627; 3,264,801; 3,501,897; 3,755,535; 3,760,565; 3,778,501; 3,832,445; 3,835,031; 3,840,643; 3,846,536; 3,892,677; 4,001,376; 4,006,066; 4,039,478; 4,153,534; 4,153,535; 4,181,705; 4,206,039; 4,218,344; 4,221,677; 4,233,276; 4,238,317; 4,241,033; 4,254,616; 4,258,020; 4,267,072; 4,300,997; 4,323,542; 4,325,811; 4,369,109; 4,369,130; 4,376,103; 4,381,991; 4,405,443; 4,423,019; and 4,443,419. These prior art methods have met with varying degrees of success.
Flue gas streams discharged from regenerators, power plants, or other combustors are commonly directed through one or more dedusters, such as flue gas scrubbers, electrostatic precipitators, cyclones, bag houses, granular bed filters, or other filters, in order to remove particulates from the flue gas stream. Typifying these dedusters and other prior art particulate-removing devices are those shown in U.S. Pat. Nos. 3,540,388; 3,550,791; 3,596,614; 3,608,529; 3,608,660; 3,654,705; 3,672,341; 3,696,795; 3,741,890; 3,769,922; 3,818,846; 3,882,798; 3,892,658; 3,921,544; 3,922,975; 4,017,278; 4,126,435; 4,196,676; and 4,421,038. These dedusters and prior art devices have met with varying degrees of success.
The combined use of flue gas scrubbers and electrostatic precipitators, while often effective to control particulate emissions, is very expensive and cumbersome.
It is therefore desirable to provide an improved process to remove nitrogen oxides, sulfur oxides, and particulates from gaseous streams.